Manufacture of magnesium alloy extrusions



Patented June 1,1954

MANUFACTURE OF MAGNESIUM ALLOY EXTRUSIONS Jay R. Burns, Ferguson, Mo.,assignor to The Dow Chemical Company, Midland, Mich., a corporation ofDelaware No Drawing. Original application April 10, 1950,

Serial No. 155,133. Divided and this application March 18, 1953, SerialNo. 343,237

2 Claims.

The invention relates to methods of making extrusions of magnesium-basealloys. It more particularly concerns an improved method of makingextrusions having high tensile strength and other desirable propertiesfrom magnesiumbase alloys in the form of atomized powder.

Recently, it has been discovered that by diexpressing atomized powderinstead of using solid ingots of a magnesium-base alloy, as in the usualextruding process, the resulting extrusions are stronger and otherwisemore desirable except that the surface of the extrusions may beblemished by innumerable blisters which form in the surface layer of theextrusions while they are still hot on leaving the die opening. In someinstances, blistering is increased by a subsequent anneal of theextrusion. Attempts to prevent the formation of the blisters bycarefully preventing air from being entrapped in the atomized powder asit is compacted into solid metal during extrusion do not overcome thediificulty. I have discovered that the cause of the blistering on makingextrusions of atomized magnesium-base alloys is the presence ofrelatively friable nonmetallic solid contaminants which are invariablypresent in conventionally atomized magnesium-base alloys. These alloysare atomized by impinging a jet of cool gas against a falling thinstream of the molten alloy. The molten alloy stream becomes broken upinto fine spherical particles which I quickly solidify. Each particle isa mass of extremely small crystals of the solid alloy. The atomizationis carried out in steel equipment in an atmosphere of natural gas whichissues from the jet and is reused. Microscopic examination of theatomized product shows the presence of up to about 0.2 percent by weightor more in some cases of nonmetallic contaminants among which are foundmagnesium oxide, magnesium hydroxide, hydrated iron oxides, magnesiumcarbonate, and hydrated magnesium chloride. The contaminants are in theform of irregular friable particles having more or less the same size asthose of the atomized metal. It appears that in some way unknown to methese contaminants give rise to gas formation in the extruded metal, thegas having suflicient pressure to raise blisters in the surface layersof the hot extrusion as it issues from the die I have discovered that bycrushing these contaminants or otherwise comminuting them so that theirparticle size is reduced Well below that of the atomized metal anddisseminating the comminuted contaminants throughout the atomized powderthe resulting mixture may be extruded into a blister fre'product. Thecomminution of the contaminants is carried out in the presence of theatomized metal-particles, the dimensions of which are not substantiallyafiected by the comminution operation. The comminuted contaminants neednot be separated from the atomized metal prior to extrusion.

The invention then consists of the improved method of making extrusionsof atomized magnesium-base alloys hereinafter fully described andparticularly pointed out in the claims.

In carrying out the invention, the mass of atomized magnesium-base alloyto be extruded is subjected to a difierential comminution in which thefriable nonmetallic contaminants of the mass are comminuted in thepresence of the atomized metal particles of the mass withoutsubstantially reducing the size of the metal particles. While thenonmetallic contaminants are reduced in particle size they are dispersedthroughout the mass of atomized metal par ticles, thereby avoiding anexcessive concentration of nonmetallic matter at any point in the massof metallic particles. The comminution of the nonmetallic contaminantswithout substantially comminuting the metal particles can be effected invarious ways as by placing the mass of atomized metal, as atomized, in aball mill, hammer or impact mill, or like device and subjecting the masstherein to the forces of comminution. Prior to the diiferentialcomminution, particles coarser than 20 mesh and finer than about 200mesh preferably are screened out and rejected. The rejected portion mayamount to about 15 to 35 per cent by weight of the atomized mass. In thecomminution operation, the forces applied are to be below a value whichwill cause comminution of the metal, yet strong enough to crush orabrade the nonmetallic contaminants and reduce their particle size todust-like dimensionsv The contaminants are much more friable than themetal particles and crush easily under comparatively light pressure toparticles much finer than the metal particles compared to that needed tocomminute the metal particles. At the same time as the mass of atomizedmaterial is being subjected to the crushing and abrading forces of thedifierential comminution operation, the resulting abraded or comminutedportion of the mass becomes therethrough, resulting in a uniform mixtureof atomized metal particles and comminuted nonmetallic contaminants.

The amount of abrasion required has no substantial effect on the sieveanalysis of the material treated. For example, a magnesium-base alloyas-atomized has the following sieve analysis before and after abradingin a hammer mill with sufficient force to crush and disperse thenonmetallic contaminants throughout the atomized powder.

Sieve Slze Wt. Percent Wt. Percent before Comafter Oom- Passlng Retainedminution minution throughon- In the above example, it will be seen thatthe differential comminution operation produces no significant reductionin the as-atomized particle sizes in general, although there is 100 percent increase in the very small proportion which passes through the 200mesh sieve.

The treated mass thus obtained is subjected to heating, compacting, andextruding, the heating being performed preferably in the container of aconventional extrusion apparatus. The temperatures employed are thoseused in conventionally extruding magnesium-base alloys, e. g. about 550F. to about 850 F. The atomized metal reaches extruding temperaturesquickly on contact with the walls of the heated extruding container andextruding may be begun within a minute or two after charging the heatedcontainer.

A quantity of a magnesium-base alloy having a nominal composition of 3per cent of aluminum, 1 per cent of zinc, 0.3 per cent of manganese, thebalance being magnesium was atomized and the atomized mass was screenedthrough a 20 mesh sieve to screen out particles larger than 20 mesh. Themass of atomized particles passing through the 20 mesh sieve wasscreened on a 150 mesh sieve and the portion passing through it wasrejected. About 1 quart of the portion of atomized powder retained onthe 150 mesh sieve was subjected to a differential comminution bypassing it through a micropulverizer (hammer mill) 3 times in successionin which all the particles were subjected to hammering, abrading andmixing, the hammering and abrading being suiiicient l was charged intothe 3-inch diameter container of an extrusion press.

The container was at 650 F. and the charge of differentially comminutedatomized powder was held in the container for 5 minutes to allow thecharge to reach 650 F. before the extrusion was begun. The heated chargewas then compacted and extruded through a rectangular die opening, x78", at the rate of about 5 feet per minute to form a flat extrudedstrip about 20 feet long. The strip was free from blemishes in the formof blisters and exhibited normal values for the mechanical properties oftensile and compression strengths.

For comparison, another quart of the screened, as atomized, powder ofthe alloy Was extruded from the same container through the same die atthe same temperature and rate of extrusion but without subjecting thescreened atomized powder to the differential comminution. The extrusionobtained had an average of 540 blisters per square foot of surface.

Example II A quantity of a magnesium-base alloy having a nominalcomposition of 1 per cent of aluminum, 1 per cent of zinc, and 1 percent of manganese, the balance being magnesium was atomized. Theatomized P wder obtained was screened. Particles coarser than 20 meshand finer than mesh were rejected. A quart of the screened portionretained on the 150 mesh sieve was subjected to the differentialcomminution as in Example I by passing the screened atomized powderthrough the same micropulverizer four times. The sotreated powder wasextruded as in Example I with the result that about a 20-foot length ofthe extrusion was obtained, the surface of which was free from blisters.

In comparison, another portion of the same screened atomized powder wasextruded in similar manner through the same die but without subjectingthe powder to differential comminution with the result that the surfaceof the extrusion had an average of more than 450 blisters per squarefoot.

This application is a division of my prior copending application SerialNo. 155,133, filed April 10, 1950, now abandoned.

I claim:

1. The method of making extrusions having a surface free from blistersof a magnesium-base alloy in the form of atomized powder, said powderconsisting of spherical particles of the alloy and nonmetalliccontaminants in the form of friable particles acquired as normalcontaminants in the process of making the said atomized powder, whichcomprises subjecting the atomized particles to a differentialcomminution in which the solid friable nonmetallic contaminants arecrushed in the presence of the metallic particles and the crushedparticles disseminated uniformly throughout the mass of metallicparticles without substantially reducing the dimensions of the metallicparticles, heating the resulting mixture of metallic particles andcrushed nonmetallic particles to extruding temperature, anddie-expressing the heated mixture to form an extrusion.

2. The method of making extrusions having a surface free from blistersof a magnesium-base alloy in the form of atomized powder, said powderconsisting of spherical particles of the alloy and nonmetalliccontaminants in the form of friable particles acquired as normalcontaminants in the process of making the said atomized powder, whichcomprises screening out of the atomized powder as made the particleslarger than about 20 mesh and smaller than about 200 mesh, particles toextrudin temperature, and die-exsubjecting the so-screened atomizedparticles to pressing the heated mixture to form an extrua differentialcomminution in which the solid sion.

friable nonmetallic contaminants are crushed in V the presence of themetallic particles and 5 References Cited in the file of this patent thecrushed particles disseminated uniformly throughout the mass of metallicparticles with- UNITED STATES PATENTS 1out substlantially reducing thedimensions of Number Name Date he metal ic particles, heating theresulting mixture of metallic particles and crushed nonmetallic l02'630623 Chlsholm et a1 1953

1. THE METHOD OF MAKING EXTRUSIONS HAVING A SURFACE FREE FROM BLISTERSOF A MAGNESIUM-BASE ALLOY IN THE FORM OF ATOMIZED POWDER, SAID POWDERCONSISTING OF SPHERICAL PARTICLES OF THE ALLOY AND NONMETALLICCONTAMINANTS IN THE FORM OF FRIABLE PARTICLES ACQUIRED AS NORMALCONTAMINANTS IN THE PROCESS OF MAKING THE SAID ATOMIZED POWDER, WHICHCOMPRISES SUBJECTING THE ATOMIZED PARTICLES TO A DIFFERENTIALCOMMINUTION IN WHICH THE SOLID FRIABLE NONMETALLIC CONTAMINANTS ARECRUSHED IN THE PRESENCE OF THE METALLIC PARTICLES AND THE CRUSHEDPARTICLES DISSEMINATED UNIFORMLY THROUGHOUT THE MASS OF METALLICPARTICLES WITHOUT SUBSTANTIALLY REDUCING THE DIMENSIONS OF THE METALLICPARTICLES, HEATING THE RESULTING MIXTURE OF METALLIC PARTICLES ANDCRUSHED NONMETALLIC PARTICLES TO EXTRUDING TEMPERTURE, ANDDIE-EXPRESSING THE HEATED MIXTURE TO FORM AN EXTRUSION.